1. Technical Field
This invention relates to restrained connections for flexible joint pipe and particularly to a restrained connection for mechanical joint pipe.
2. Background Art
Gray iron and ductile iron pipes in nominal sizes ranging from 2 inches to 48 inches are extensively used in systems handling water and other liquids at pressure ratings from 150 to 350 pounds per square inch. Conventional connections for such pipes are of three general types: bell-and-spigot (or slip-on), flanged, and mechanical joint.
A bell-and-spigot connection is the simplest and least expensive; it is made up by inserting a plain end of one pipe into a bell end of another pipe or fitting. The joint is traditionally sealed by lead calking, but in recent years rubber gaskets have become more popular.
In a flanged connection, each end of a pipe or fitting has an integrally cast or a threaded-on flat-faced flange, and the joint is made up by bolting together the flanges of two abutting pipes, or a pipe and a fitting, with a flat rubber gasket between the flange faces.
A mechanical joint is a bolted joint of the stuffing box type and comprises three elements--a flanged bell, a rubber sealing gasket, and a compression ring or gland. The bell may be cast as an integral part of one end of a standard length of pipe or a fitting (e.g., a tee, elbow, etc.), or it may be cast as a separate unit and threaded for assembly onto a threaded end of a pipe, in the same manner as a threaded flange. The bell end of one pipe section accepts a plain end of another pipe section of the same nominal size, after first slipping a flanged gland and a rubber gasket over the plain end. The gasket fits inside the bell and is maintained in compression by bolting the flange of the gland to the flange of the bell.
A principal advantage of bolted flange connections is that the bolts prevent axial separation of the joint; however, such a connection will not accommodate any angular misalignment. Both bell-and-spigot and mechanical joint connections, on the contrary, can tolerate varying degrees of angular misalignment, depending on the pipe diameter, but friction of the gasket provides the only restraint against separation of the joint. Although the compression exerted by the gland of a mechanical joint connection increases the frictional resistance, as well as improving the sealing effectiveness, of this type of joint as compared with a rubber-gasketed bell-and-spigot connection, massive concrete foundations or other restraining means are required to prevent separation by internal pressure at locations where a pipeline changes direction.
Various types of locking devices are available for restraining slip-on or mechanical joint connections, but these devices either convert the joint into a rigid connection, thereby losing the angular flexibility which is the principal advantage of such connections, or the devices require a substantial amount of precision machining, which greatly increases their cost.
Examples of locking devices are shown in the American Pipe Manual, Fifteenth Edition, 1979, published by American Cast Iron Pipe Company, Birmingham, Ala., and include retainer rings welded to the outside of a plain end pipe to prevent the pipe from slipping through a gland ring bolted to a flanged mating bell end pipe or fitting (pages 9-2 to 9-6), setscrews threaded radially through a mechanical joint gland into the outside of plain end pipe (pages 9-10 to 9-12), and setscrews threaded through a plain end pipe from inside and engaging a groove machined inside a mating slip-on bell (pages 9-15 to 9-16). Double-ended adapter glands are also available, for mating assembly between opposed bell ends of mechanical joint pipe (page 9-7. See also Clow Corporation catalog "Pipe Economy," published 1975, at pages 85-92). These restraining devices permit pipe to be laid with initial angular deflections, but do not accommodate subsequent deflections due to settling after installation. To retain flexibility after assembly, it is necessary to use ball joint pipe (pages 10-1 to 10-21 of American catalog or pages 55-59 of Clow catalog). Ball joint pipe is expensive, however, because the surfaces of the ball and mating socket must be accurately machined.